1. Field of the Invention
The invention relates to precisely controllable electronic drive circuits and more specifically to electronic circuits for energizing a d.c. load from an a.c. source in proportion to the value of a d.c. control signal.
2. Description of the Prior Art
Situations arise wherein it is necessary to drive an electrical load from an a.c. source in proportion to the magnitude of a control signal.
In gyroscopic systems used aboard aircraft or marine vessels, for instance, two axes of gimbal freedom are often provided, which in the normal erect position of the gyroscope are orthogonal to each other and to the gyroscope rotor spin axis. When erect, the spin axis is parallel to the gravitational vector. However, when the gyroscope is to be started or when the spin axis otherwise becomes displaced from its normal position, the spin axis must be erected into a position parallel to the gravitational vector.
In general, such systems employ direct current torque motors mechanically coupled to the respective gimbal axes to provide erecting torques in response to a signal derived from means for sensing deviations of the spin axis. The sensing means frequently consists of an electrolytic vertical reference switch containing an electrolyte disposed between a mounting plate and a shell so as to form a bubble. Metallic detection plates contact the liquid, and as the gyroscope deviates from the erect position, the bubble moves so that the resistance of the device changes accordingly. Such a sensor is described, for instance, in U.S. Pat. No. 3,020,770, issued to W. W. Young et al on Feb. 13, 1962. A control signal is derived from the sensing means and applied to a torque motor drive circuit wherein it is converted to a signal suitable for driving the associated torque motor. Many prior art systems, such as the system described in the aforementioned U.S. Pat. No. 3,020,770, employ servomotors which require bulky and heavy inductive devices such as Scott-T transformer networks which limit their utility in situations where space and weight are at a premium.
Other systems are available which operate on a pulse principle. U.S. Pat. No. 3,604,276, issued to Charles Eliot Hurlburt on Sept. 14, 1971, for instance, includes a multivibrator for generating a rectangular pulse train. The pulse train is used to switch power to the torque motor in response to control signals from a sensor. The pulse system permits the gyroscope to be torqued at a rate greater than the normal erection rate, and prolongs switch life by limiting current through the switching means. Although the pulse repetition rate and pulse duration may be adjusted to match the characteristics of a given system, there is no suggestion of varying either of these parameters as a function of variations in the control signal, as contemplated in the present invention.